Include mavenCentral in java build.gradle (#1653)

the android dep is currently missing from jcenter. This adds
mavenCentral as an additional repository to the default java generator's
build.gradle.

Author: jimschubert

modules/openapi-generator/src/main/resources/Java/build.gradle.mustache

@@ -6,6 +6,7 @@ version = '{{artifactVersion}}'
 
 buildscript {
     repositories {
+        mavenCentral()
         jcenter()
     }
     dependencies {
@@ -15,6 +16,7 @@ buildscript {
 }
 
 repositories {
+    mavenCentral()
     jcenter()
 }